This section provides background information related to the present disclosure which is not necessarily prior art.
A “flexible package” is normally made of very flexible and conformable materials that do not have a fixed shape; it can conform to the product it is holding, may stretch around the product, and is typically puncture resistant. Flexible packaging is popular because forming a flexible package is simple, unfilled packages require little storage space, even filled packages occupy less overall volume than a rigid alternative, and finally flexible packaging materials offer excellent display graphics possibilities. Display graphics are important because the flexible package usually is a consumer product, and the package itself becomes the prime label. Display graphics are usually printed by a flexographic process or a gravure process. It is important that constituents of the ink NOT end up in the interior of the package where it could contaminate the product; this could happen by means of migration through the packaging materials or by offsetting in the rewound roll (the rewound roll is an intermediate step of forming the flexible package).
Flexible packages are usually laminated structures, i.e., unique materials either co-extruded or adhesive bonded to achieve a set of performance attributes, such as puncture resistance, water vapor permeability, oxygen permeability, ink migration, adhesive migration, heat seal ability, etc. Laminated or co-extruded structures can be used pre-constructed before printing of the display graphics, but then there would be ink exposed which may not resist damage from mechanical contact and/or which may migrate or offset and find its way to the interior of the package. Most often, the display graphics are reverse printed on the interior side of the package, and then a barrier material is laminated over the graphics to protect the graphics and to prevent ink migration or offsetting. Where adhesive is used, migration of the adhesive must also be taken into consideration.
The bond between the layers comprising the laminate usually needs to be stronger than the laminate materials themselves, and one of the materials will fail during delaminating before the adhesive fails. There are different adhesive systems available which are water-based, solvent-based, and two-part epoxy. The two-part epoxy is known more commonly as solventless and is the primary system used in flexible packaging because it yields the highest bond strength—a destructive bond.
Solventless lamination conventionally begins by mixing the two components of the epoxy. When mixed, a chemical reaction begins which polymerizes the adhesive into a solid over a period of time. The period of time that the mixed epoxy can be handled in application machinery is termed the pot life. After the epoxy is mixed, it is applied to the base substrate, after which a laminate material is applied and the construction then wound into a roll. The bond strength at the initial lamination is termed the green bond, which is a fraction of the final bond strength, that final bond strength requiring as much as 72 hours to achieve. The green bond strength, immediately after laminating, typically is not enough to allow the construction to be run through slitters.
Pot life, green bond strength, and final bond strength are related, and a longer pot life typically means a reduced green bond strength and longer time to final bond strength. Ideally, pot life would be infinite and green bond strength would immediately equal final bond strength, but the practical pot life requirements of the operation results in low green bond strengths and about a 72 hour time to final bond strength.
Solventless lamination is usually a separate operation from printing because the pot life of the mixed epoxy is shorter than the press changeover time. If the laminator is stopped for a press changeover, it would have to be cleaned up to prevent curing of the epoxy in the equipment, while in an offline process, the laminator runs continuously and seldom stops.
Thus, a typical process involves printing display graphics and rewinding; queuing up the solventless laminator, laminating the layers and rewinding; queuing up the slitter, (waiting for sufficient bond strength), and slitting and rewinding; and queuing up for shipment (waiting for final bond strength).